Re: [PATCH] parisc: Fix csum_ipv6_magic on 32-bit systems

[Guenter Roeck <linux@roeck-us.net>]

On 2/11/24 05:57, David Laight wrote:
> From: Guenter Roeck
>> Sent: 10 February 2024 19:16
>>
>> Calculating the IPv6 checksum on 32-bit systems missed overflows when
>> adding the proto+len fields into the checksum. This results in the
>> following unit test failure.
>>
>>      # test_csum_ipv6_magic: ASSERTION FAILED at lib/checksum_kunit.c:506
>>      Expected ( u64)csum_result == ( u64)expected, but
>>          ( u64)csum_result == 46722 (0xb682)
>>          ( u64)expected == 46721 (0xb681)
>>      not ok 5 test_csum_ipv6_magic
>>
>> This is probably rarely seen in the real world because proto+len are
>> usually small values which will rarely result in overflows when calculating
>> the checksum. However, the unit test code uses large values for the length
>> field, causing the test to fail.
> 
> Isn't length limited by the protocol encoding?
> So this is really a bug in the unit tests for using a length that
> it too large for the function?
> 

Arguable. While the length value passed to the function is not a valid
packet length, it exposes a weakness in the implementation of
csum_ipv6_magic() - after all, folding proto+len into the checksum _may_
overflow even for small(er) values of proto and length. It is just much
less likely to happen if the length is limited to 16 bit.

Guenter